High voltage switch assemblies with sub-atmospheric or vacuum type circuit interrupters for electric power circuits and systems are well known in the art, such as is shown in U.S. Pat. Nos. 4,568,804; 3,955,167 and 3,471,669. Encapsulated vacuum type switches or circuit breakers are also known, as is shown in U.S. Pat. Nos. 3,812,314 and 2,870,298.
In such switch assemblies and circuit breakers, a pair of coacting contacts, one fixed and the other movable, are provided for controlling and interrupting current flow. The contacts are provided in a controlled atmosphere contact assembly which also includes a relatively fragile glass or ceramic housing, commonly referred to as a "bottle". The contacts are housed within the bottle. A metal bellows is typically provided on one end of the bottle, and the movable contact is linked to the inside of the bellows. An operating rod attached to the outside of the bellows can be moved so as to move the movable contact inside the bottle. The interior of the bottle is maintained under a controlled atmosphere, such as air or another gas under a low sub-atmospheric pressure, to protect the contacts from damage caused by arcing when the contacts are opened and closed. The glass or ceramic wall of the bottle provides a permeation-resistant enclosure which maintains the controlled atmosphere for the life of the device. While efforts have been made as is shown in the above mentioned patents to protect and reinforce such contact assemblies with solid dielectric materials surrounding the bottles, there are considerable needs for further improvements.
In particular, there is a significant, unmet need for an elastomer-insulated switch using a controlled atmosphere contact assembly, which would be suitable for underground power distribution systems and other, similar applications. Switches for use in such applications must meet several demanding requirements. Those parts of the switch assembly connected to line voltage during use, including the contact assembly and operating rod, must be encased in a solid insulating housing having dielectric strength sufficient to withstand the maximum voltage which may be imposed on the system, which may be tens of thousands of volts for a distribution-level system. For safety, the insulating housing should be covered with a conductive layer that can be grounded. The switch should be operable from outside of the dielectric housing, without opening the housing. It should be capable of withstanding many years of exposure to temperature extremes, water and environmental contaminants. The switch must also survive continued exposure to high voltages. The switch should withstand repeated operation. To minimize arcing during switch opening and closing, the switch should include a mechanism to move the contacts rapidly, while also limiting the forces applied to the contacts and to the bottle as the contacts open and close. The switch should also be manufacturable at reasonable cost.
U.S. Pat. No. 3,471,669 seeks to provide such a switch for underground applications. The switch according to the '669 patent includes a sub-atmospheric or vacuum type controlled atmosphere contact assembly. The contact assembly for the coacting contacts has spaced reinforcing rods about the exterior and is directly encapsulated in a generally waterproof elastic jacket made of a "suitable synthetic resin substance such as one of the elastomers, silicone rubber or epoxy rubber", covered by an electrically conductive coating for grounding. A snap acting toggle assembly is disposed inside the jacket and linked to the operating rod of contact assembly. A rotatable shaft of dielectric material extends from the exterior of the jacket to the toggle assembly. Rotation of the shaft actuates the toggle to move the contacts and close or open the circuit.
However, the switch described in the '669 patent has not been widely adopted in the art. As reported in Odom et al, Development and Testing of Encapsulated Vacuum Sectionalizing Switch for Underground Distribution (IEEE publication, date unknown), elastomers which are vulcanized under heat and pressure cannot be used readily to form the housing in the switch design and manufacturing process as shown in the '669 patent. The pressures encountered in molding such elastomers cause breakage of the bottles incorporated in the contact assemblies. The elastomer tends to penetrate into the mechanism, and to cause other problems.
Certain elastomers vulcanized by heat and pressure are especially useful insulating materials for underground electrical power systems. Elastomers such as EPDM (ethylene propylene diene monomer) combine high dielectric strength with excellent resistance to the effects of ozone and corona discharge. These elastomers can also provide good physical properties such as abrasion resistance, and can be molded at reasonable cost. Additionally, these elastomers can be compounded with conductive additives and molded to provide an electrically conductive grounding layer integral with the dielectric housing. For these and other reasons, elastomers molded and vulcanized under heat and pressure, such as EPDM, have been almost universally adopted as materials of construction for the housings used in other underground electrical distribution systems. The inability to use elastomers vulcanized under heat and pressure represents a serious shortcoming of the switches and methods disclosed in the '669 patent and Odom et al. article.
Additionally, the rotatable shaft extending through the jacket of the devices shown in the Odom et al. article and '669 patent poses serious reliability problems. Such a movable interface is susceptible to contamination and dielectric failure.
Perhaps for the foregoing reasons, the switches and methods disclosed in the '669 patent and Odom et al. article have not been widely adopted in the industry despite the long-felt need. Indeed, despite the long-felt need for a suitable polymer-insulated switch for underground high voltage systems, no truly satisfactory answer has been found heretofore.